During the 1980s and 1990s, second generation (2G) cellular communication systems were implemented to provide mobile phone communications. 3rd generation (3G) cellular communication systems have been widely installed during the past decade or so, to further enhance the communication services that may be provided to mobile phone users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) and Frequency Division Duplex (FDD) or Time Division Duplex (TDD) technology.
FDD means that the transmitter and receiver, in a given device or base station, operate at different carrier frequencies. Uplink (UL) and downlink (DL) frequencies/sub-bands are separated by a frequency offset. FDD can be efficient in the case of symmetric traffic such as voice and as a consequence many historical spectral allocations are paired for FDD operation.
A full-duplex system, allows communication in both directions to/from a base station, and, unlike half-duplex, allows this to happen simultaneously. Land-line telephone networks are full-duplex, since they allow both callers to speak and be heard at the same time. Two-way radios can be, for instance, designed as full-duplex systems, which transmit on one frequency and receive on a different frequency.
A half-duplex system provides for communication in both directions, but only one direction at a time (not simultaneously). Typically, once a party begins receiving a signal, it must wait for the transmitter to stop transmitting, before replying. An example of a half-duplex system is a two-party system such as a “walkie-talkie” style two-way radio, wherein one user must indicate an end of transmission, and ensure that only one party transmits at a time, as both parties transmit on the same frequency, sometimes referred to as simplex communication.
A recent development in 3G communications is the long term evolution (LTE) cellular communication standard, sometimes referred to as 4th generation (4G) systems, which are compliant with 3GPP™ standards, which will be deployed in existing spectral allocations owned by Network Operators and new spectral allocations yet to be licensed. Irrespective of whether these LTE spectral allocations use existing 2G and 3G allocations being re-farmed for fourth generation (4G) systems, or new spectral allocations for existing mobile communications, they will primarily be paired spectrum for FDD operation.
In TDD systems, the same carrier frequency is used for both uplink (UL) transmissions, i.e. transmissions from the mobile wireless communication unit (often referred to as wireless subscriber communication unit) to the communication infrastructure via a wireless serving base station and downlink (DL) transmissions, i.e. transmissions from the communication infrastructure to the mobile wireless communication unit via a serving base station. In TDD, the carrier frequency is subdivided in the time domain into a series of time slots and/or frames. The single carrier frequency is assigned to uplink transmissions during some time slots and to downlink transmissions during other time slots. In FDD systems, a pair of separated carrier frequencies is used for respective uplink and downlink transmissions to avoid interference therebetween. An example of communication systems using these principles is the Universal Mobile Telecommunication System (UMTS™).
Typically, a wireless subscriber unit is ‘connected’ to one wireless serving communication unit, i.e. a base station serving one communication cell. Transmissions in other communication cells in the network typically generate interfering signals to the wireless subscriber unit. Due to the presence of these interfering signals a degradation of the maximum achievable data rate, maintained to the wireless subscriber unit, is typical. Such interference is often referred to as ‘inter-cell’ interference.
However, within the communication cell, a wireless subscriber communication unit may also observe/be affected by interference from other wireless subscriber communication units communicating within the same cell. Such interference is often referred to as intra-cell interference.
The duplex spacing, between uplink (UL) and downlink (DL) frequency carriers, and the duplex gap, i.e. the frequency separation between the uplink and the downlink band edges, often varies across the spectral allocations of FDD systems. In some instances, both the duplex spacing and the duplex gap can be set to be very narrow, for example with the duplex spacing ˜=2× channel bandwidth and the duplex gap ˜=1× channel bandwidth. In these instances half duplex operation of the mobile is desirable or more often as not essential, as it is practically impossible to achieve the required radio frequency (RF) filtering to perform full duplex FDD in a form factor that is compatible with the decreasingly small size of a communication handset.
The primary reason for this phenomenon in a handset is that a duplexer (radio frequency separator/filter) that allows simultaneous uplink and downlink operation, without interference from the other, is physically not realisable for cost and/or size reasons. In effect, the duplexer must, in the transmit path, essentially filter the adjacent channel leakage rejection (ACLR) emissions of the transmitter such that that the handset's transmissions leaking into the handset's receiver chain are well below the noise floor of the receiver. Furthermore, the duplexer must also, in the receive path, filter the transmit (UL) signal (in-band and not out-of-band), so that it does not block the receiver.
Referring now to FIG. 1, a graphical example 100 of the aforementioned FDD HD problem is illustrated with regard to transmit power 105 versus frequency 110. The UL transmit band 115 is shown as being adjacent the DL receive band 120. Within these frequency bands, paired narrowband UL transmit channels 135 and DL receive channels 140 are allocated, with UL channel transmissions that fall out-of-band for the downlink receive channel being filtered 125 to an acceptably low power level by the receive filter. UL adjacent channel emissions falling in-band for the downlink channel are filtered 130 to an acceptable level by the transmit filter. The paired nature of the narrow duplex Transmit-Receive separation (duplex spacing) 135 is also illustrated.
In the past, half-duplex systems, which were typically narrowband in nature, have relied on a combination of channel filtering (at baseband frequencies) and radio frequency (RF) band-filtering in the receiver in order to provide sufficient selectivity to protect against inter-user interference when two users are in close proximity. Thus, in HD FDD systems, one communication unit is scheduled UL resource in a first time slot whilst, for the same time slot, a second communication unit may be scheduled the DL resource. Taking, as an example, the Terrestrial European Trunked RAdio (TETRA), allocated spectral bands at 400 MHz, there are 2×5 MHz allocations with a 10 MHz duplex spacing between uplink and downlink carriers or 5 MHz duplex gap. The TETRA system operates with a narrow channel bandwidth of 25 kHz across the 2×5 MHz spectral allocations, so in this scenario the downlink channel is many channels away from the corresponding uplink channel. Thus filtering for full duplex operation is feasible.
However, in considering a 5 MHz deployment of an LTE system in this spectral band, the downlink channel will reside in the second adjacent channel of the uplink. The default second adjacent channel performance of an LTE user equipment (UE) is similar to that of a UMTS™ UE of the same bandwidth at −43 dBc. If the transmit power of the UE is +23 dBm, this means that the adjacent channel power, without any special filtering measures, is −20 dBm. The noise floor of a reasonable UE is typically around −100 dBm. Thus, in order to cause less than a 3 dB noise rise, the interference must also be at or below this level. Thus, a significant 80 dB additional selectivity (or RF signal rejection) is required from any duplexer or series of filters.
If, however, the use of TETRA in the 400 MHz band is replaced by a broadband data-oriented system, such as HD-FDD LTE, with a channel bandwidth of 5 MHz then filtering to the appropriate level 10 MHz away is much more difficult, if not impossible to achieve, given the current and projected state of the art in filter technology.
In a base station, where the size and cost of a duplexer is acceptable, the base station would still operate in full duplex communication utilising all of the available frequency/time resource. In the base station the aforementioned level of RF filtering can be achieved with machined metal cavity filters, sometimes using dielectric resonators, which today cost around US $500 and have a significant size at sub-1 GHz frequencies. Note that because the elements of these filters are proportional to the carrier wavelength their size increases with decreasing frequency. Thus, such types of components would never be suitable for small form factor handset style devices, whereas full duplex operation is still acceptable in a base station.
FIG. 2 illustrates an example of the timing 200 of full duplex FDD UE communications 200 and half duplex FDD communications 240. As illustrated, the full duplex FDD UE communication 200 allows allocation of simultaneous UL transmit 210 from, and DL receive 220 time slots to, the UE. As illustrated, the Half Duplex FDD UE communications 240 do not allow allocation of simultaneous UL transmit 250 and DL receive 260 time slots by a base station scheduler.
In a HD-FDD system the scheduler has the responsibility for not simultaneously allocating the same uplink and downlink slots to a given user. This solves the intra-user interference problem (i.e. the scheduler can ensure that the UE does not transmit and receive at the same time (or at least schedules time slots to allow sufficient switching time between transmit and receive operation).
However, there is a possibility of an inter-user problem between two handset devices if one user of a first handset is allocated a downlink slot whilst another user of a second handset in close proximity to the first handset is allocated the same uplink time slot. This inter-user problem is less serious than the potential intra-user problem due to the coupling loss (in addition to any duplexer attenuation). However, it has been appreciated that this can still present a serious problem if the interferer is transmitting at high power and the victim handset is at a communication cell edge.
Consequently, current techniques are suboptimal. Hence, an improved mechanism to address the potential inter-user interference problem, for example for a scenario where a new broadband system may be deployed in an historical spectral allocation traditionally used by a narrowband system, would be advantageous.